Communications systems that employ a shared communications medium may, in certain circumstances, permit two or more users to transmit information at the same time. For example, a wireless receiver picks up a number of packets, asynchronously transmitted by different users/terminals. The receiver contains multiple antennas. The packets have random relative delays and their signals are offset in frequency due to oscillator imperfections and Dopper. In addition, each transmitted signal is exposed to phase noise also due to oscillator imperfections. A packet consists of preamble, which is known and same for all packets, and payload, which contains information symbols.
These asynchronous transmissions can cause multiple transmitted packets to overlap in time upon arrival at a designated receiver, which gives rise to packet collision. At the receiver, such overlapping transmissions combine to form a composite signal. When a collision occurs, the multiple transmissions interfere with each other in a manner that can prevent the reception of a portion or all of the information in these transmissions.
Recovery from a collision requires the retransmission of information by users. Unfortunately, as collision probability increases, so does the expected number of the retransmissions. If the number of retransmissions becomes excessive, latencies associated with the transfer of information increase and communications capacity is wasted.
To reduce the amount of wasted capacity, many communications systems are able to adjust their parameters to optimize performance. For example, certain TDMA systems are able to dynamically adjust the number of allocated contention time slots to keep collision rates, often measured in collisions per second, within an acceptable range.
Thus, to effectively control collision rates, a communications system needs to accurately detect collisions. Conventional collision detection techniques do not provide great accuracy. For example, one such technique detects collisions based solely on received signal power. According to this technique, a collision is detected when one or more power measurements are above a certain level.
This power-based technique disadvantageously assumes that all signals have the same power and detect collision by detecting jumps in the power of the received signal. However, normal transmit power variations, as well as gains of different channels that different packets are transmitted over, may corrupt such collision detection process. In addition, frequency offsets between two or more colliding signals can result in a combined signal that does not have an increased power level within the observation window, thereby leading to an inaccurate estimates of the time instant at which collision occurs. Finally, the power based technique fails to detect a substantial number of collisions in a way that such failures result in collision rate overestimation or underestimation, which leads to ineffective contention slot allocation decisions.
Traditionally, a packet collision problem is addressed by forcing the communication system to avoid packet collisions. In short, if collision has been detected, the terminals are forced to back off certain time from transmission and one of the terminals would get the resource and retransmit its packet.
However, these conventional mechanisms for collision avoidance schemes suffer from latency and overall system capacity reduction. Latency results from forcing the terminals to back off and retransmit packets after a certain time period. Capacity reduction is caused by not using the channel resources optimally. Additionally, collision avoidance mechanisms are embedded into MAC and network layer of the protocol stack, which also contributes to latency and capacity reduction.
Accordingly, there is a need for systems and methods to determine the collision in the presence of frequency offsets, timing offsets and an angle of arrival, as well as estimates a channel gain corresponding to the transmission of each packet over a channel in the common communication medium.